Method of concentrating aqueous rubber dispersions



Patented Dec. 12, 1933 1,939,635

UNITED: STATES PATENT orrrcs iua'rnon or ooncnm'aarmo AQUEOUS RUBBER DISPERSIONS Henry B. Townsend, Belmont, Mam, assignor to Vultex Corporation of America, Cambridge, Mass. a corporation of Massachusetts No Drawing. Application April 23, 1931 Serial No. 532,283

11 Claims. (CI. 18-50) This invention relates to an improved method to vulcanization, and rubber practically devoid of concentrating an aqueous vulcanized rubber such natural proteins is'commonly vulcanized dispersion and to the concentrated material proonly after suitable proteins are returned to it. duced by the method. Furthermore, such processes while removing pro- I The invention has for one of its objects to pro tein, water-solubles, or both, frequently require .50

vide a simple, quick, highly effective and comthe addition of materials that may be equally as mercially feasible method of making a highly harmful to the desired irreversible properties of concentrated, purified and stable vulcanized rubthe latex rubber. ber dispersion which may be used for many pur- The removal of protein, without adding some poses in its concentrate or paste form or may be foreign protective colloid, is a removal of the transported and stored cheaply and conveniently natural protective or stabilizer for the latex. in concentrated form and later diluted to the re- While such purified latex may be equally as stable quired solid content by the addition of water. as regards friction, it shows exceedingly high Various methods are known by which rubber instability when brought in contact with sulphur,

18 latex or rubber latex preserved with ammonia zinc oxide or accelerators as commonly used for may be thickened or concentrated for more convulcanization. I have discovered that by submitvenient use, for greater facility and less expense ting latex to a vulcanizing operation, I can then in transportation and for certain applications remove the undesirable protein and water-soluble where a thickened or more viscous material or materials in a centrifugal of the general type '0 heavier solid content isindicated. Creaming used for purifying oils, and obtain a purified and agents have been added to commercial latex causirreversible vulcanized rubber upon evaporation ing a separation of the material into layers, the of the dispersing phase. By repeated operations, upperlayer containing the larger proportions of I am able to secure substantially'pure and comthe rubber solids and the lower layer containing 'mercially stable concentrates which have 60% 25 the larger proportion of the serum. By drawing to 65% total rubber solids. By a single operation, off the upper layer, a material is recovered having I have produced a material comprising 60% total a rubber solid content much greater than the vulcanized rubber solids. average 30% rubber solid content of normal latex. In carrying out my invention according to a By this method, however, much of the protein, preferred method, ordinary commercial rubber 80 resin and water soluble matter is carried over latex as received from the plantation having a to the recovered concentrate together with some solid content of approximately 30-40% and of the creaming agent dissolved in the cream. without additional anti-coagulant or foreign Latex has been thickened by the addition of zinc stabilizer is poured in a steam jacketed containoxide. This does not effect atrue concentrate er, a water paste is prepared containing four 86 but merely thickens and increases the viscosity. parts of sulphur, one-half part of zinc oxide, and 90 Concentration has been proposed by thermal one-half part of piperidonium penthamethylene evaporation methods, and by coagulating in the dithiocarbamate dispersed in water of approxipresence of an anti-coagulant whereby thickening mately four times the weight of the other mate'- & or some agglomeration may be efiected without rials. The mixture is added to the latex in the complete coagulation. container and with slow agitation is gradually A film produced by evaporation of the -dispersbrought to a temperature of 168 F. over a period ing phase of latex is partially reversible, i. e. such of one hour. This temperature is maintained latex rubber, spots with water, turning white. for an hour and then the materialis force cooled Such rubber is readily injured by even normal to 142,F. over a period of one-half hour. The I 45 humidity changes. This spotting is most noticelatex is'now vulcanized and is a substantially difable on the upper surface of the film as dried, ferent material from ordinary rubber latex. and is caused by impurities present in the serum; "Some of the difference between vulcanized latex Some of these impurities are sugars or waterand ordinary latex are exhibited by a change in solubles, while others are of a protein nature. the particle size and-shape as seen under an.

50 The removal of these materials has been proposed ultramicroscopc. Vulcanized latex does not have 1 previously, as by dialysis after heating with causa, pronounced tacky characteristic as found in tic. Such methods are not well adapted to comlatex and even when air dried in a film gives a mercial work, and such purified rubber is not rubber which is far less tacky than latex. Also suitable for vulcanization. It is recognized that ,vulcanized latex exhibits a much higher surface 55 the natural proteins of rubber are a distinct aid tension than ordinary latex. When tested by treatment with chloroform, the vulcanized material changes to a loose curd while latex is converted to a coherent, sticky ball or gob. Vulcanized latex is rendered substantially sterile by the vulcanizing treatment and in this respect is distinguishable from latex.

The vulcanized latex is then allowed a settling period in a sedimentation tank where the excess After the clarification processes, the cured;

latex, at 30% to 40% total solid content, is in-'" troduced to a centrifugal, preferably of the De Laval type. By controlling the input rate to the machine, a concentrate of 60-70% rubber solids may be obtained and likewise a dilute phase of 8-12% solids. The rubber in the concentrated phase has been well separated from approximately 65% of natural protein, approximately 75% of the water solubles, and approximately 30% of the resins. A large proportion of the protein, water extract and resins (acetone extract) is found in the dilute .phase.

After this concentration, the concentrate phase is diluted with water and ammonia added to replace that lost until the material resembles the condition in which it was originally introduced to the centrifugal machine, and the diluted material is passed through the centrifugal machine at such an input speed that the concentrated phase recovered will have approximately 60-70% total solids. In this second operation, it

v is found that the dilute phase contains approximately 4-6% solids. In certain cases, it may be desirable to again dilute the concentrate phase of the second concentration with ammonia and water and again pass through the centrifugal machine whereby a concentrate phase in the neighborhood of 60-70% total solids is again obtained, but whereby the dilute phase will be appreciably lower in total solid content and may be obtained as low as 1% total solids.

The dilute phases from each of the separate concentrating runs can be combined and the solids which are rubber hydrocarbon can be recovered largely by further centrifugalization. Most of the ammonia in the original material as vulcanized, as well as that added during the washing or diluting processes, is removed by the concentrating operations. If desirable, ammonia or other volatile preservatives may be added to the final product, although the material appears commercially stable and may be transported long distances without any tendency toward coagulation.

A film produced by evaporation of the dispersing phase from the concentrated material does not water-spot to a milky color and is high in characteristics of di-electric strength, elasticity the non-rubber constituents carried by said se- "of the vulcanized rubber particles and a dilute rum while maintaining said particles in a dispersed condition, removing practically all of the proteins and water soluble constituents together with excess of vulcanizing agents and side reaction products, from the dispersion of rubber particles by washing them with water and ammonia, and, throughout the process, maintaining the serum and washing liquid free from nonrubber constituents other than those originally contained in the vulcanized latex.

2. That improvement in methods of purifying vulcanized rubber latex which consists in centrifuging the vulcanized latex and thereby divid ing it into a concentrate phase containing most phase-containing a high proportion of the nonrubber constituents, diluting the concentrate phase so produced, repeating the centrifuging operation to effect a further separation of the vulcanized rubber particles from the liquid, and, throughout the process, maintaining the serum and washing liquid free from non-rubber constituents other than those originally contained in the vulcanized latex.

3. That improvement in methods of purifying and concentrating vulcanized rubber latex which consists in centrifuging the vulcanized latex and thereby dividing it into a concentrate phase containing most of the vulcanized rubber particles and a dilute phase containing a high proportion of the non-rubber constituents, washing the rubber particles in the concentrate phase so produced, separating the washed rubber particles from surplus serum and washing liquid and the non-rubber constituents carried by them, avoiding the addition of non-rubber constituents other than ammoniacal water to either the serum or the concentrate phase at any stage of the process, and preventing any substantial coagulation of the rubber particles at any stage of the process.

4. That improvement in methods of purifying aqueous dispersions of rubber which consists in vulcanizing the rubber particles in the dispersion while maintaining them in a dispersed condition, separating most of the non-rubber solids introduced by the vulcanizing step from the other constituents of the dispersion, centrifuging the remaining dispersion under controlled conditions to separate most of the vulcanized rubber particles from a considerable proportion of the serum and the non-rubber constituents carried by said serum, diluting the concentrate phase containing the vulcanized rubber particles so separated, repeating the centrifuging operation to effect a further separation of the rubber particles :30 from the surplus liquid and the non-rubber constituents, preventing any substantial coagulation of the rubber particles at any stage of the process, and avoiding the addition to said serum or said concentrate phase of any protective colloid, or creaming agent, at any stage of the process.

5. A purified vulcanized latex free of protective colloids and from which the suspended protein and all water soluble and water attracting constituents, and excess of vulcanizing agents, to- 14 gether with side reaction products have been substantially removed.

6. A purified vulcanized latex, commercially stable, but free of protective colloids, and practically free of proteins and water soluble constituents, and from which excesses of vulcanizing agents, together with side reaction products have been substantially removed, and containing not more than a negligible proportion of the resins natural to the original latex.

7. A purified vulcanized latex tree of protective colloids and from which excesses of vulcanizing agents together with side reaction products, have been substantially removed, and which will dry practically free of constituents soluble in water or capable of adsorbing or absorbing water.

8. A purified and concentrated vulcanized lavoid of constituents soluble in water or capable of taking up water.

10. That improvement in methods of purifying and concentrating vulcanized rubber latex which consists in centrifuging the vulcanized latex and thereby dividing it into a concentrate phase containing most of the vulcanized rubber particles and a dilute phase containing a high proportion of the non-rubber constituents, maintaining said particles in'a dispersed condition during said centrituging operation, and avoiding the addition of any protective colloid to either the serum or said concentrate phase at any stage or the process.

11. That improvement in methods of purifying rubber latex, which consists in first vulcanizing the latex and then separating most of the vulcanized rubber particles from part of the serum in which they are suspended and removing a high proportion oi. the non-rubber constituents carried by said serum, while maintaining said particles in a dispersed condition, by centrifuging the vulcanized latex and dividing same into dilute and concentrate rubber phases, removing practically allof the proteins and water soluble constituents together with excesses of sulphur and HENRY B. TOWNSEND. 

